Physicians rushing to get much-needed oxygen to a patient in a medical emergency commonly prescribe pure oxygen. But a new study suggests the therapy may trigger a release of chemicals that actually make it harder for the heart to deliver oxygen.

A team at the University of California, Los Angeles, led by neurobiologist Ronald Harper, examined functional magnetic resonance imaging (fMRI) of brain activity in 14 healthy children ages 8 to 15 as they inhaled normal air, 100 percent oxygen and a mixture of 95 percent oxygen and 5 percent carbon dioxide.

After the children breathed pure oxygen, researchers recorded increased activity in the hippocampus and hypothalamus. The oxygen–carbon dioxide mix caused far less activity, and in some cases none at all, according to the study, published in May in Public Library of Science (PLoS) Medicine.